Shame on all of you! Not a single person has yet to mention Inuyasha!
I NEVER SKIP an opening or closing credits (I guess I'm extremely rare in that regard). It is my belief that these segments are useful for creating an emotional pallete (Intros are usually happy/upbeat, closings are usually sad). All of my friends hate me for never skipping (if I have the remote

That being said there are only 2 openings that have had an impact on my "opening hating" friends, and they've both been mentioned already. The first is Ranma 1/2 season 1. Seeing Akane with long hair dancing and shaking her behind like its nobody's business is not only hypnotic, but its very catchy along with the music. After seeing it (in a group of about 6 people) about 4 times, we were all dancing along with Akane, shaking it.
The second is the breathtaking Lodoss War TV intro. I kid you not the show is only worth buying for its intro, which seems to be done by a completely different company (production I.G.?). The video is set to kiseki no umi (sea of Miracles) and is simply cinematic art.
I'd say I really enjoy nearly 90% of credit music/video. There are several great closing credits, but the most noteworthy "My Will," the closing credits to Inuyasha season 1. I actually had an initial dislike for Inuyasha through the first 13 episodes. The only thing that kept me watching was the beautiful song "My Will" and accompanying cinematography which literally made me tear up the first 10 times I saw it.
I am so thankful that it kept me hooked, the creation of one of my favorite characters ever occurs in episode 14, and the series seemingly gets even better and ever better with every episode after that. Inuyasha is now my 2nd favorite anime (and I've seen a lot). Although I was kinda bummed when they switched My Will with Fukai Mori.
Anyway, intros:
Inuyasha Opening 1
Inuyasha Opening 2
Ranma 1/2 Opening 1
Ranma 1/2 Opening 2
Ranma 1/2 Opening 3
Full Metal Panic
Chobits
Neon Genesis Evangelion
Love Hina
Love Hina Again
Lodoss War TV
Escaflowne
Onegai Teacher
Onegai Twins
R.O.D.
Outlaw Star
Closing Creditis:
Inuyasha Season 1
Inuyasha Season 2
Inuyasha Season 3
Inuyasha Season 4
Inuyasha Season 5
Ranma 1/2 Season 1
Ranma 1/2 Season 2
Ranma 1/2 Season 3
Cowboy Bebop
Full Metal Panic
Love Hina
Love Hina Again
Escaflowne
Here are the "worst" ones:
Intro:
Um, none really.
Well maybe Trigun, for going way too heavy on the Heavy Metal Guitar, but it was ok in my book.
Berserk was laughably bad at first (medieval swordsman to Bob-Dylan-like anonymous rocker?) But it eventually got catchy when my friend started making hand-fart noises to it. After seeing berserk twice all the way through, I would say it finishes as a decent opening.
Oh here we go! Noir's Intro was TERRIBLE! It actually hurt to hear it. The sound shrieks at times and the corresponding video is dark and neon colored at the same time.
Closing Credits
Evangelion's closing credits were to a Frank Sinatra song that was sung by everyone Except Frank Sinatra. Also, At least a few of them sounded like Barbara Streisand (nuff said).
X- Decent song, but you are just zooming out on a still picture with some feathers passing. Laziest intro/closing ever, by far.
Berserk: Whats the sound of one demon Moaning? Maooo!!! Again fodder for my friend's hand fart noises. The 2nd berserk ending credits are much improved.
Ranma 1/2: Hate to say it, because Ranma 1/2 is by far my favorite show ever, but when you run as long as Ranma does, your gonna have some pretty crappy intros and Credits. Season 7 intro is notably bad. Some closing credits were pretty bad too.